The present invention relates to a method for deriving a standard 12-lead electrocardiogram effective for diagnosing ischemic heart disease, acute myocardial infarction, or the like, in which a minimum number of electrodes are attached to predetermined areas on the body surface of a living body to thus obtain an electrocardiographic waveform, and focus is placed on an ST segment of the electrocardiographic waveform which will serve as an important index of, in particular, ischemic heart disease. The present invention also relates to an electrocardiograph using such a method.
Conventionally, when an electrocardiogram of a patient is detected, measured, and recorded in a hospital or a like facility, a total of ten electrodes are attached to the body surface of the patient; namely, six positions for chest leads, and four positions for limb leads. Six limb-lead waveforms (I, II, III, aVR, aVL, and aVF) of standard 12-lead waveforms and six chest-lead waveforms (V1, V2, V3, V4, V5, and V6) of the same are derived from electric potentials of the heart detected and measured by the ten electrodes by measuring means, such as an electrocardiograph.
The related-art electrocardiograph or the like can detect, measure, and record an electrocardiogram which is formed from standard 12-lead waveforms and allows appropriate diagnosis and treatment of a variety of heart diseases, through use of ten electrodes. Such a diagnosis and treatment using a plurality of electrodes is possible in a fully-equipped hospital, or the like, where a patient is maintained at rest. However, when at-home or emergency medical treatment is to be performed, no time is available for attaching a large number of electrodes to appropriate positions on the body surface of the living body, from the viewpoint of the status of the patient; moreover, difficulty is encountered in transmitting a large number of lead waveforms in the form of multi-channel signals. In addition, since only one channel (i.e., one lead) or a like number of channels can radio-transmit an electrocardiogram signal, a heart disease is diagnosed through use of, at most, two to four electrodes.
More specifically, conventionally, the following limitations in obtaining a standard, at-rest 12-lead electrocardiogram are pointed out. Namely, (1) attachment of six electrodes to the chest of a patient inhibits a clinician from accessing the chest of the patient; (2) in some cases where a wound is present and/or a bandage is applied onto the chest of the patient, the clinician cannot attach all six chest electrodes; (3) electrodes, wires, and amplifiers required for collecting 12-lead electrocardiogram data over 12 channels increase the overall cost of the apparatus; and (4) the volume of data for representing the 12 channels of 12-lead electrocardiogram data exceeds a maximum capacity; in other words, a transmission bandwidth of a general, typical telemeter unit.
From the above viewpoints, the following configuration has been conventionally practiced as means for detecting and recording an electrocardiogram of standard 12-lead waveforms with a small number of electrodes. For example, four special positions (four electrodes of EASI) on the chest surface of a living body are used, and respective electrocardiographic waveforms thereof are lead. Once signals of the electrocardiographic waveforms have been converted into a vectorcardiogram with use of a fixed coefficient, the thus-converted vectorcardiogram is converted into a 12-lead electrocardiogram. The thus-obtained electrocardiogram is known as an EASI-lead electrocardiogram.
In the lead method of the EASI electrocardiogram of the related art, an approximation to a standard 12-lead electrocardiogram can be attained to a certain degree. However, when leads from the four special positions on the chest surface of the living body are emplaced, appropriate attachment of the electrodes to respective specified positions encounters difficulty, since health care professionals, such as doctors and nurses, are not clinically accustomed to this attachment work, thereby posing a problem of variation arising in detection accuracy of the electrocardiogram. In addition, as described above, when arithmetic operation is performed to acquire the 12-lead electrocardiogram from the electrocardiographic signals derived from the electrodes, the signals must be converted twice (from EASI leads to a vectorcardiogram, and from the vectorcardiogram to a 12-lead electrocardiogram) through use of the fixed coefficient. Accordingly, in some cases variation arises in calculation accuracy. Furthermore, since none of the leads are actually measured values of the 12 leads, some doubts arise with regard to reliability.
General relationships among lead waveforms and measurement positions shown in FIGS. 4A and 4B, and potentials for obtaining a 12-lead electrocardiogram are as follows.
TABLE 1IvL − vRIIvF − vRIIIvF − vLaVRvR − (vL + vF)/2aVLvL − (vR + vF)/2aVFvF − (vL + vR)/2V1v1 − (vR + vL + vF)/3V2v2 − (vR + vL + vF)/3V3v3 − (vR + vL + vF)/3V4v4 − (vR + vL + vF)/3V5v5 − (vR + vL + vF)/3V6v6 − (vR + vL + vF)/3
Accordingly, in the lead method for the EASI lead electrocardiogram of the related art, positions to which the EASI electrodes are attached for measuring respective potentials are special and differ from those of the measurement positions of the lead waveforms of the case shown in Table 1. Therefore, accuracy in positioning to the specified positions in attachment of the electrodes poses considerable influences to a measurement result, which is inconvenient, in that the attachment requires rich experience, and the like. In addition, even when a patient is resting in a fully equipped hospital, or the like, the number of the electrodes used for the standard 12-lead measurement is large. Accordingly, problems arise not only in terms of inconvenience for the patient, but also in terms of increased load on a health care professional who applies the electrodes.
From the above viewpoints, Japanese Patent Publication No. 2002-34943A proposes a method and an electrocardiograph for deriving a standard 12-lead electrocardiogram which enables appropriate diagnosis and treatment of a variety of heart diseases by making use of a lead system subset constituted of the minimum number of leads for obtaining a conventionally-known standard 12-lead electrocardiogram or an M-L leads (Mason-Likar leads) electrocardiogram; and filed a patent application therefor.
Specifically, this method utilizes, as a lead system subset constituted of the minimum number of channels, for instance, leads I and II of limb leads, and leads V1, and V5 or V6 of chest leads which have been used for obtaining the standard 12-lead electrocardiogram. By virtue of the configuration, leads III and aVs (leads aVR, aVL, and aVF) are calculated on the basis of intrinsic relationships among the leads shown in Table 1. The remaining chest leads V2, V3, V4, and V6 or V5 are calculated on the basis of relationships between the potential matrix, the lead vectors and the heart vectors.
A standard 12-lead electrocardiogram obtained as above utilizes the lead system subset of the related-art standard 12-lead electrocardiogram. Therefore, in attachment of the electrodes, positioning to the respective specified positions can be performed easily and without fail without requiring rich experience for the work. Hence, a highly-accurate standard 12-lead electrocardiogram can be derived, thereby enabling appropriate diagnosis and treatment of a variety of heart diseases.
Japanese Patent Publication No. 2002-282229A discloses another method for solving the problem. Specifically, a plurality of electrodes, in a number fewer than ten, are attached to a patient; the respective electrodes are placed at any of 12-lead positions of a standard 10-lead electrodes; electric signals are collected from the electrodes; and a 12-lead electrocardiogram is generated on the basis of the collected electric signals. More specifically, there is disclosed a device which utilizes multiple-linear regression with use of expansion-coefficient equations, to thus arithmetically generate leads in a number fewer than twelve. There is also disclosed that the expansion-coefficient equations are determined on the basis of any of 12-lead electrocardiograms of a hospital's general population, those of a sub-population of the hospital's general population, or those having been previously acquired from the patient.
According to the technique disclosed in Japanese Patent Publication No. 2002-34943A, complicated calculation is required for: calculation of potential matrix and the lead vectors from the minimum leads; estimation of heart vectors on the basis of potential-lead vectors; and further obtaining a standard 12-lead electrocardiogram from the heart vectors.
On the other hand, according to the technique disclosed in Japanese Patent Publication No. 2002-282229A, a standard 12-lead electrocardiogram is obtained by the multiple-linear regression with use of the expansion-coefficient equations. The expansion-coefficient equations are determined by comparing data obtained through uniform sampling of every portion of electrocardiogram at a rate of 120 to 1,000 per second. By such a method, average accuracy can be achieved in derivation of a standard 12-lead electrocardiogram. However, clinically, particularly in ischemic heart disease, acute myocardial infarction, and the like, a change of 0.1 mV in an ST segment of an electrocardiogram is considered to be of pathological significance. Accordingly, as a monitoring diagnostic apparatus for use in pre-hospital care, and the like, emphasis must be placed on accuracy of the ST segment.